In Part 1, we briefly covered how to optimize power consumption in capacitive sensing system, utilizing sensor ganging with a capacitive sensing controller, and use cases of sensor ganging. In Part II, we reviewed tradeoffs between response time and power consumption, as well as other issues resolved by a ganged sensor. In this final segment, we will cover sensor ganging implementation and more techniques to optimize power consumption.
Sensor ganging implementation
Sensor ganging implementation with a MCU/controller such as Cypress’ PSoC4 involves primarily four steps:
- Selection of sensors to be ganged
- Checking parasitic capacitance of sensors and re-ganging of sensors if required
- Capacitive sensing component settings for a ganged sensor
- Firmware implementation
Selection of sensors to be ganged
There may be applications where not all sensors need to be ganged to detect the approach of a human hand approach or first touch. This needs to be considered while ganging the sensors. If it is enough to gang just a subset of sensors, this reduces the parasitic capacitance of the ganged sensor and farther optimizes power consumption.
Checking parasitic capacitance of sensors and re-ganging the sensors if required
Find out the parasitic capacitance of each individual sensor (which are part of a ganged sensor) using the appropriate API provided by the CapSense component in the PSoC Creator IDE. The parasitic capacitance of the ganged sensor is approximately the sum of the parasitic capacitances of the individual sensors that are ganged. If the parasitic capacitance of the ganged sensor is beyond the maximum supported parasitic capacitance by the CapSense component (per the datasheet), increase the number of ganged sensors and split the sensors to be ganged intelligently among the new ganged sensors so that the parasitic capacitance of all the ganged sensors are within the maximum limit.
CapSense component settings for a ganged sensor in PSoC Creator
In a PSoC4 application, in order to gang sensors, a dummy sensor has to be placed in the CapSense component. The dummy sensor selected should be a proximity sensor. Once a proximity sensor is placed, select the “number of dedicated sensor elements” for the proximity sensor as ‘0’. This is because this dummy sensor is just used to gang the sensors and not to be used as a dedicated sensor.
Once the dummy sensor is placed, the component now allows the user to select sensors to be ganged for the dummy sensor. The user can select the sensors based on the application need and based on the parasitic capacitance of the ganged sensor as shown in the figure below. If there is need for more than one ganged sensor, place as many dummy sensors as ganged sensors in your application.